In the field of wireless communication, for many applications, such as within a cellular radio infrastructure, medical and/or industrial applications, etc, radio frequency (RF) power amplifiers (PAs) are required to supply an increasing amount of output power. In order for RF PAs to achieve such an increasing power capacity demand, it is necessary to use several individual die blocks in a parallel structure. This necessity is primarily due to the aspect ratio of RF transistors (namely their high length to width ratio) as well as mounting technology limitations.
A problem encountered when two or more die blocks are arranged in parallel is that the devices are often prone to odd mode current instabilities. At high operating frequencies, for example in an operating region of 2 GHz, the die size becomes significant compared to the wavelength of the operating frequency, leading to the creation of unbalanced phase combinations between both the parallel die blocks and RF PA loops which, with the presence of any noise at the input of the amplifier (transistor), may lead to oscillations.
FIG. 1 illustrates schematically an example of an RF PA circuit 100 comprising a first power amplifier block 110 and a second power amplifier block 120 operably coupled in parallel as is known in the art, for example as may be used within RF integrated circuits (RFICs). Each power amplifier block 110, 120 comprises an input matching circuit 130, 140 operably coupling an RF input signal 105 to a respective single-ended RF PA transistor 112, 122. In this context, the term ‘single-ended’ refers to a common mode amplifier and not a differential mode, such as a push-pull amplifier. An output of the drain port of each RF PA transistor 112, 122 is operably coupled to an output matching circuit 150, 160 to attempt achieving a maximum power transfer of the RF PA transistors 112, 122.
As shown, signals present at the input 105 of the RF PA circuit 100 are able to follow two different paths to an output 170 of the RF PA circuit 100, as illustrated in the equivalent circuit of FIG. 2 as Path ‘A’ 210 and Path ‘B’ 220. Typically, these two paths 210, 220 indicate how both signals and noise can be combined or cancelled within the RF PA circuit 100. For example, at a particular frequency and if Path ‘A’ 210 and Path ‘B’ 220 are phase matched, common mode noise at the input of the RF PA circuit 100 will be combined at the output of the RF PA circuit 100, whereas differential mode noise at the input of the RF PA circuit 100 will be cancelled at the output of the RF PA circuit 100. However, due to the physical sizes of the RF PA devices, for example for devices that are required to support a 100 W output power, it is extremely unlikely that both paths will be phase matched. Furthermore, at a particular frequency, and if Path ‘A’ 210 and Path ‘B’ 220 are phase mis-matched by 180 degrees, common mode noise at the input of the RF PA circuit 100 will be cancelled at the output of the RF PA circuit 100, whereas differential mode noise at the input of the RF PA circuit 100 will be combined at the output of the RF PA circuit 100. It is desirable to avoid the combination of differential mode noise in such situations, which may cause reflections that in turn produce oscillations and instabilities within the RF PA circuit 100.
FIG. 3 illustrates a common technique for removing such instabilities within an RF PA circuit 300, in particular a technique employed within RFICs. The drain ports 314, 324 of the RF PA transistors 312, 322 are interconnected to one another via a resistor 310. The interconnection of the drains using resistors in this manner modifies (in a lossy manner) the differential odd mode currents that are intrinsic to any common mode amplifier, and substantially removes much of the related current instabilities mentioned above. The gates (inputs) 316, 326 of the RF PA transistors 312, 322 may also be interconnected to one another via a resistor (not shown) in a similar manner.
Although resistors are generally simple to implement within integrated circuits, in the case of RF PA integrated circuits, the use of resistors to interconnect the drains and gates of the power transistors requires an additional component technology, namely in the form of resistor component technology, requiring a significant amount of additional space and manufacturing complexity.
A known alternative solution to the problem of common mode instabilities mentioned above is to interconnect the drains and gates of the power transistors using inductors, or simply wires, in place of the resistors. The purpose of the inductors is to homogenize the differential odd-mode currents between the transistor blocks, stabilising the odd-mode currents without the need for additional component technology (e.g. resistors). However, the use of inductors in this manner is typically only sufficient for removing odd mode current instabilities at some areas of the frequency spectrum, in particular at higher frequencies. In particular, inductors within integrated circuit packages are typically provided by lengths of wire, with the inductive values being controlled by the length and diameter of the wire. Thus, for high power base station PAs, a circuit design able to remove GHz oscillations would require inductors in the 1 nH to 2 nH range with radio frequency capacitors used for decoupling, i.e. 100 pF to 200 pF MOSCAPS. Lowering the frequency of oscillation would typically translate to increasing the value for the inductors and capacitors that are used. The inductance value is limited in these cases due to its wire bonding implementation. Hence, the capacitor value would need to be increased to the μF range, which is unfeasible using current manufacturing technology.
As a result, it is difficult to calibrate inductances without significantly impacting the layout, etc. of components within the IC package. Accordingly, the use of inductors in this manner significantly limits the amount that a power amplifier integrated circuit (PAIL) layout can be tuned. As a result, a particular PAIL is typically only able to provide stability over a limited frequency range.